1. Field of the Invention
The present invention relates to a method and apparatus which can be used to better disperse a gas into a liquid. The dispersed gas can be soluble in the liquid, in which case the improved dispersion aids in dissolution of the gas into the liquid. The dispersed gas can be immiscible in (or have low solubility in) the liquid and can be used to strip (remove) dissolved gas or other high vapor pressure component from the liquid, in which case the improved dispersion of the stripping gas improves the stripping rate. In addition, there are applications wherein it is desired to react a gas with a liquid or with a reactant which is dispersed in the liquid, in which case the improved dispersion of the gas in the liquid increases the reaction rate between the gas added to the liquid and the liquid or between the gas and the other reactant present in the liquid. The functional portion of the apparatus for dispersing the gas is preferably fitted to the inside of a pipeline so the gas dispersion can be accomplished while the gas-liquid mixture is traveling through the pipeline.
2. Background of the Invention
Various methods of dissolving a gas in a liquid are known in the art. Given sufficient surface area and contact time, and a liquid which is not saturated with the gas under the existing temperature and pressure conditions, the gas can be dissolved in the liquid. Typically the gas is introduced into the liquid in a manner which provides for good mixing between the gas to be dissolved and the liquid, to improve the efficiency of the dissolution.
U.S. Pat. No. 3,778,038 to Eversole, dated Dec. 11, 1973, describes a method of distributing fuel in air followed by passing the air and fuel mixture through a constricted zone to increase the velocity of the mixture to sonic. Downstream from the constricted sonic zone, the air and fuel mixture is accelerated to supersonic velocity in a supersonic zone. Thereafter, the mixture is decelerated to subsonic velocity in a subsonic zone to produce a shock zone where the fuel droplets entrained in the air are believed to be further subdivided and uniformly distributed throughout the combustible mixture before the mixture is supplied to an engine cylinder.
U.S. Pat. No. 4,639,340 to Garrett, dated Jan. 27, 1987, discloses a method for dissolving gas in a liquid comprising pressurizing a stream of liquid, introducing the gas into the pressurized stream to form a gas-liquid mixture traveling at a velocity less than the velocity of sound in the liquid, accelerating the pressurized mixture stream to a supersonic velocity to create a first shockwave effective to reduce the size of the gas bubbles in the stream, thereby forming a dispersion of the gas under pressure in the liquid, reducing the velocity of the stream below sonic velocity, and transporting the pressurized stream to an outlet near which the velocity of the pressurized stream is accelerated to supersonic velocity once more to create a second shockwave prior to exit of the stream from the outlet.
U.S. Pat. No. 4,743,405 to Durao et al., dated May 10, 1988 discloses an apparatus for injecting a gas such as carbon dioxide into a flowing liquid. The liquid is contained in a three-section conduit consisting of converging, bottle-neck and diverging sections. The sections are joined end-to-end without concordance radii so as to produce a turbulent effect in the liquid flowing thorough the conduit. An annular chamber is disposed about the bottleneck section and includes a connection to a source of pressurized gas or the like. The annular chamber is separated from the bottleneck by a wall perforated with micro-holes which allow gas entrainment into the flowing liquid. A plurality of hollow needles which extend varying amounts into the liquid flow are disposed in the wall to provide multiple sites of gas injection in the annular chamber.
The present invention utilizes several of the techniques discussed above to introduce gas into a liquid, but provides an improved method and apparatus which improves the efficiency of the gas dissolution into the liquid.
The removal of a gas or a high vapor pressure component dissolved or held in suspension in a liquid by contacting the liquid with a stripping gas is also known in the art. The requirement for removal of the dissolved gas or high vapor pressure component is that its partial pressure in the stripping gas be lower than the partial pressure of the dissolved gas or high vapor pressure component in the liquid, thus permitting mass transfer from the liquid into the stripping gas. The rate of removal from the liquid is controlled by the rate of diffusion from the liquid into the stripping gas phase. The dissolved gas or high vapor pressure component in the liquid reaches equilibrium with the dissolved gas or high vapor pressure component in the stripping gas in time, after which no additional diffusion from the liquid occurs. The conditions for equilibrium may be described by Henry's Law.
A good description of typical equilibrium phenomenon and of the kinds of parameters involved was presented in the Proceedings of the 17th Congress of European Brewing Convention, pp. 245-58, 1979, in an article by J. Hoggan et al., entitled "A Simple Production System For The De-oxygenation of Water". This article describes a method of removing (stripping) dissolved oxygen from brew using nitrogen gas as the stripping gas. United Kingdom Provisional Patent Specification No. 1,156,417 of Harold Davidge and Charles Sydney Gibbs, published June 25, 1969, describes a method of removing dissolved oxygen from liquids by bubbling through the liquid a gas having a lower partial pressure of oxygen than that of the liquid, which gas does not react chemically with the liquid. A sparging device is used to feed the stripping gas, nitrogen, into the liquid. The oxygen removal rate is substantially increased by causing turbulent flow of the liquid stream at a point downstream of the point of introduction of the nitrogen gas. Different devices capable of creating turbulence are described.
There are numerous applications for stripping gases out of liquids, one of the most common being stripping oxygen from liquids. Typical examples, not intended to be limiting, follow. In enhanced oil recovery, sea water is injected directly into oil wells. It is necessary to remove dissolved oxygen from the sea water prior to its use to minimize corrosion. The de-oxygenated water typically has less than 0.1 to 1 ppm of dissolved oxygen. In most cases, the oxygen removal is effected by vacuum de-aeration or hydrocarbon stripping. U.S. Pat. No. 4,017,276 to Bloem, dated Apr. 12, 1977, discloses a method of stripping dissolved oxygen from sea water using a nitrogen stripping gas. Sea water is introduced into a stripping tower, and as the result of countercurrent contact between the nitrogen stripping gas and the sea water, oxygen concentrations as low as 7.5 to 12 ppm remain in the sea water after stripping. U.S. Pat No. 4,612,021 to Palmer, dated Sept. 16, 986, and United Kingdom Patent Application GB 127711, dated Apr. 18, 1984, disclose methods of de-oxygenating seawater used in water flood petroleum recovery. United Kingdom Patent GB 1531537, issued Nov. 8, 1978, discloses a method of using nitrogen gas to remove dissolved oxygen from sea water. The nitrogen is placed into cocurrent flow with the sea water and then mixed into the sea water using an in line static mixer; the mixture is subsequently fed into a cyclone separator.
De-oxygenation of boiler feed water has typically been carried out in thermal de-aerators by raising the temperature of the water with steam A. Beevers, Process Engineering of London, Vol. 66, No. 8, p. 41, August 1985, in an article entitled "Cool Way to De-aerate", reported using nitrogen as a stripping gas to remove oxygen and carbon dioxide from boiler feed water. Japanese Patent JP 60/183012 discloses a method for removing dissolved oxygen in boiler feed water by nitrogen stripping.
Japanese Patent JP 56/121681 discloses a method of removing oxygen from the water in an open cooling system used in a tire plant. The cooling water is sparged with nitrogen at a nitrogen:water ratio of 6:1.
Japanese Patent JP 59/154109 discloses a method of sparging industrial water with nitrogen to reduce the oxygen content of such water to less than 0.1 ppm dissolved oxygen.
Japanese Patent JP 58/133885 describes a method of removing oxygen from process water by sparging with nitrogen at reduced pressure.
U.S. Pat. No. 2,413,102 to Ebert, et al., dated Dec. 24, 1946, describes a method of removing dissolved gases from liquids or solutions. In general, the invention involves the commingling of a gaseous stream, such as of air and a stream of the solution to be de-gassed by forcing them together through one or more constrictions in a conduit opening into a region of reduced pressure, such as into a vessel in which a vacuum is maintained.
U.S. Pat. No. 3,132,013 to Kumamoto et al., dated May 5, 1964, describes a process for treating feed water, including the removal of oxygen from the water using a nitrogen stripping gas.
U.S. Pat. No. 3,732,668 to Nichols, dated May 15, 1973, discloses a system for inserting aircraft fuel tanks, whereby dissolved oxygen is removed from the fuel.
U.S. Pat. No. 4,352,682 to Kemp, Jr. et al., dated Oct. 5, 1982, describes a de-oxygenating unit for removal of oxygen from water which is to be used to produce carbonated beverages.
U.S. Pat. No. 4,259,360 to Venetucci et al, dated Mar. 31, 1981 describes a method of reducing dissolved oxygen content of foodstuffs and of water used in beverage production by gas sparging with nitrogen.
Most of the art cited above demonstrates the stripping of a first gas from a liquid using a second stripping gas. The objective is to obtain maximum removal of the first gas while minimizing power consumption as well as consumption of the stripping gas. It is also possible to remove particulate contaminants from a liquid using a stripping gas. The particulates may adhere to the stripping gas itself or may adhere to a volatile component within the liquid which is removed from the liquid by the stripping gas.
Stripping gas consumption has been reduced by generation of better mixing between the stripping gas and the liquid. The improved mixing may increase the available surface area of the stripping gas, and improves distribution of the stripping gas throughout the liquid. Either of these actions increases mass transfer of volatile components and/or particulates from the liquid into the stripping gas. However, the increased mixing has been achieved by creating turbulence between the stripping gas and the liquid, by methods such as in-line static mixers and the use of mixing chambers containing insoluble reticulated material through which the stripping gas-liquid mixture must pass. Increased turbulence has been obtained at the cost of increased pressure drop across process lines and mixing chambers, necessitating higher operating pressures, which tend to force the first, dissolved gas back into the liquid and to increase power consumption.
It would be desirable to find a method of increasing the effective surface area of the stripping gas which does not generate significantly increased system operational pressures (with accompanying detriment to separation and increased power consumption costs). The method and apparatus of the present invention provide an improvement over the known art which improves the overall efficiency in dispersion of the stripping gas.